Hydraulic systems are known for converting shaft mechanical power into fluid mechanical power via hydraulic pumps. The fluid mechanical power may be used to actuate hydraulic actuators such as linear hydraulic cylinders or rotary hydraulic motors, to perform work against a load. Shaft power for operating a hydraulic system may be provided by a combustion engine that is configured to convert chemical energy, stored in a fuel, into shaft mechanical power.
Variable displacement hydraulic pumps are known in the art. A swashplate actuator may be used to vary the volumetric flow rate of a variable displacement pump, even at a constant operating speed of the variable displacement pump. The swashplate actuator may be fluidly coupled to a hydraulic fluid outlet of the variable displacement pump, such that increasing discharge pressure at the outlet of the variable displacement pump may act to decrease the displacement, and therefore volumetric flow rate, of the variable displacement pump.
U.S. Pat. No. 7,165,397 (the '397 patent), entitled “Anti-Stall Pilot Pressure Control System for Open Center Systems,” purports to address the problem of engine stall caused by excessive hydraulic pump load applied to an engine by a hydraulic pump. The '397 patent describes a hydraulic system including an engine coupled to a main hydraulic pump and a fixed-displacement pilot pressure pump. The pilot pressure pump of the '397 patent is fluidly coupled to an anti-stall valve via an orifice.
If the demanded hydraulic power exceeds the available engine power, the torque demands of the main pump will slow the engine of the '397 patent. The decrease in engine speed decreases the pilot flow produced by the pump, and thus decreases the pressure drop across the orifice. When this differential pressure is no longer large enough to overcome the bias of an actuator spring, the anti-stall valve will switch to its at-rest position. In this position, all pilot pump flow is directed to a tank through a relief valve, and the pressure in the downstream pilot control circuits is also dumped to the tank. When the engine speed recovers sufficiently, the increased pilot flow through the orifice returns the anti-stall valve to an open position thereby restoring pilot fluid pressure to the downstream pilot control circuits.
However, the hydraulic circuit proposed by the '397 patent is complex and potentially expensive. Further, total removal of hydraulic load resulting from operation of the anti-stall valve of the '397 patent may result in jerky operation of implements and operator frustration. Accordingly, there is a need for improved hydraulic systems and methods to address the aforementioned problems and/or other problems known in the art.
It will be appreciated that this background description has been created to aid the reader, and is not to be taken as a concession that any of the indicated problems were themselves known in the art.